Conventionally, a wireless communication system operates in a proprietary radio frequency (RF) spectrum in which base stations and wireless terminals communicate through the proprietary RF spectrum licensed to a wireless operator. Recently, there have been discussions of wireless communication systems expanding usages into a free spectrum which is also known as an unlicensed spectrum, such as the Industrial, Scientific and Medical RF spectrum (ISM band) or other spectrums that are not proprietary and thus are free for public uses. The possibilities of Long Term Evolution (LTE) or LTE-advanced communication systems expanding into the unlicensed spectrums have drawn attention from telecommunication equipment vendors and operators. One reason for such interest is the potential overcrowding of licensed spectrums. In order to provide high throughput services to more users, expansions into unlicensed spectrums might deliver benefits for wireless communication systems.
In 2014 September, a new study item, namely “Study on Licensed-Assisted Access using LTE” was approved for investigations related to 3GPP Release 13. The framework for Licensed-Assisted Access (LAA) to unlicensed spectrum has also been known as Unlicensed LTE (LTE-U), which may potentially be a key feature for the next generation cellular system.
Challenges related a conventional cellular system operating in unlicensed spectrums would include operations within an environment of uncertainty as well as other potential co-existing issues. As the communications is conducted in unlicensed or shared spectrum, there might be other communications devices (using the same radio access technology or different radio access technology) that would like to use the same unlicensed spectrum or have already been using the same unlicensed spectrum. For example, communications of a LTE system may need to operate under uncertain behaviors of other devices which are not under the domain of the control of the LTE wireless system. More specifically, a LTE communication system might need to co-exist with Wi-Fi radios.
In an unlicensed band, a device under the domain of a LTE system might not be able to transmit or receive at any given time because of the spectrum sharing. This is different from the conventional cellular LTE operation in which the spectrum is licensed for transmission or receiving and thus the LTE system has an exclusive control of the licensed spectrum. In order to cope with the uncertainty, the current LTE communication system would need to undergo modifications in order to co-exist with devices that are not under the domain of the LTE communication system. In addition to modifications to cope with such uncertainty, transmissions or receptions of signaling messages in the unlicensed spectrum would need to be efficiently designed.
To be more specific, one potential challenge associated with spectrum sharing among different radio access technologies could be related to channel measurements. Although a base station may use its licensed band to communicate essential information a user equipment (UE), as long as the unlicensed spectrum is to be used, channel measurements of the unlicensed spectrum between a base station and a UE would need to be performed. For example, a base station may typically transmit a reference signal or a pilot signal which is to be received by a UE. The UE may then perform a channel measurement based on the reference signal or the pilot signal. The reference signal could be, for example, the channel state information reference signal (CSI-RS). After acquiring the CSI-RS, the UE may then measure the CSI which could then be fed back to the base station. However, as both LTE-U devices and Wi-Fi devices may generally perform channel measurements to be transmitted back to a base station or a Wi-Fi access point (AP) by using the same unlicensed spectrum, interferences may occur if spectrum sharing is accomplished in an uncontrolled manner.